Can't help, since I don't know what the "switch key" circuit is supposed to do?

 

MOD NOTE: Moved to Homework Help.

 

Leaving aside things like your 74xx244 chip being unpowered, this is a pretty straightforward analysis problem.

What does the voltage at the input of the logic chip need to be in order to be seen as a 0? As a 1?

What is the voltage at the input of the logic chip provided by each of the two circuits for the switch in each position?

 

Don't leave the base on transistor Q8 is floating. Pull down the base with a 1 kΩ resistor.
The common emitter configuration of Q7 is an inverter. The common collector configuration is non-inverting and provides no voltage gain. You don't need Q7.

 

please use English on this forum.
are you trying to debounce switch signal?

 

The first thing to consider is which circuit, if either, even attempts to do what you want.

Do you want the LED to be on when the output of the '244 is HI, or when it is LO?

 

Don't leave the base on transistor Q8 is floating. Pull down the base with a 1 kΩ resistor.
The common emitter configuration of Q7 is an inverter. The common collector configuration is non-inverting and provides no voltage gain. You don't need Q7.

You are really amazing. Your observation is very smart. I am very thankful to you.

 

[اقتباس="MrChips، المنشور: 1954735، العضو: 62020"]
لا تترك القاعدة على الترانزستور Q8 عائمة. اسحب القاعدة لأسفل باستخدام مقاومة 1 كيلو أوم.
تكوين الباعث المشترك لـ Q7 عبارة عن عاكس. تكوين المجمع المشترك غير عاكس ولا يوفر أي مكسب للجهد. لا تحتاج إلى Q7.
[/يقتبس]

What do you think of it now? Are the values of the two 10k resistors good? I put 74244 to fix the output j perfectly. The switch is a toggle switch. Does it require an anti-rebound circuit?. And thanks

 

Why transistors?

 

Why transistors?

I'm assuming (since the TS doesn't seem inclined to say) that is is part of the assignment. If so, it would be nice to know what the assignment actually is.

 

What do you think of it now? Are the values of the two 10k resistors good? I put 74244 to fix the output j perfectly. The switch is a toggle switch. Does it require an anti-rebound circuit?. And thanks
View attachment 338244

What do you mean by "perfectly"? A lot of people assume that logic gates output "perfect" logic values and design circuits that rely on that. They don't, particularly TTL. That was one of the first mistakes that I made as a student working with digital logic.

As for the switch debouncing, that depends on whether you require a clean signal. You are using a Schmitt-trigger input buffer, so that will help some, but switch bounce can go well beyond what the hysteresis of a Schmitt-trigger can handle. How bouncy is your switch? How tolerant to signal bounce is whatever is being driven by the '244?

What are the requirements of this assignment?

 

Doing what you want requires no components other than the switch, and possibly a resistor.

 

I would use no transistors.

Because this is a learning and thinking experiment:
Look at the schematic of how a TTL inverter or buffer is built. The output stage is built to pull down hard to a low lever. They do not pull up well. The output stage has a transistor whose Emitter is sitting on ground. (top choice)
Look at the voltage levels you need to have for TTL. A "0" must be below 0.7V while a "a" only needs to be above 2V.

 

I would use no transistors.

Because this is a learning and thinking experiment:
Look at the schematic of how a TTL inverter or buffer is built. The output stage is built to pull down hard to a low lever. They do not pull up well. The output stage has a transistor whose Emitter is sitting on ground. (top choice)
Look at the voltage levels you need to have for TTL. A "0" must be below 0.7V while a "a" only needs to be above 2V.
View attachment 338308

It's becomes even more clear when one realizes that a TTL input is actually a current-sourcing "output" -- current must be drawn from an input by the previous output to effect a "0" on the input.

 

Doing what you want requires no components other than the switch, and possibly a resistor.

Well... that depends on the requirements of the assignment.

Aside from that, there's also the question of whether debouncing is needed.

 

It's becomes even more clear when one realizes that a TTL input is actually a current-sourcing "output" -- current must be drawn from an input by the previous output to effect a "0" on the input.

Then, more amazingly, TTL outputs are great at sinking current, but pretty wimpy at sourcing current.

 

Then, more amazingly, TTL outputs are great at sinking current, but pretty wimpy at sourcing current.

Not amazing (or at least not surprising). The two go hand-in-hand. TTL outputs are intended to drive TTL inputs. Your outputs need to sink all the current that all of the inputs they are driving are sourcing, but since the inputs don't sink current, the outputs don't need to source much. When you want to drive something other than TTL inputs, you should use a suitable driver with TTL-compatible inputs.

 

since the inputs don't sink current, the outputs don't need to source much.

Yes.
The source current mainly just needs to drive the parasitic circuit capacitance of the traces and the driven logic circuit inputs to give the desired signal rise-time.

 

Not amazing (or at least not surprising). The two go hand-in-hand. TTL outputs are intended to drive TTL inputs. Your outputs need to sink all the current that all of the inputs they are driving are sourcing, but since the inputs don't sink current, the outputs don't need to source much. When you want to drive something other than TTL inputs, you should use a suitable driver with TTL-compatible inputs.

Certainly amazing to people with no prior familiarity with plain TTL cirvcuitry.